The present invention relates to the art of recovering petroleum, and, in particular, to the improvements in the art of enhanced oil recovery techniques.
When a well is completed in a subterranean reservoir, the oil present in the reservoir is normally removed through the well by primary recovery methods. These methods include utilizing native reservoir energy in the form of water or gas existing under sufficient pressure to drive the oil from the reservoir through the well to the earth's surface. This native reservoir energy most often is depleted long before all of the oil present in the reservoir has been removed from it. Additional oil removal has been effected by secondary recovery methods of adding energy from outside sources to the reservoir either before or subsequent to the depletion of the native reservoir energy.
Soluble or miscible phase displacement techniques comprise a form of enhanced recovery in which there is introduced into the reservoir through an injection well a fluid or fluids which are soluble in or miscible with the reservoir oil and serve to displace the oil from the pores of the reservoir and drive it to a production well. The soluble or miscible fluid is introduced into the injection well at a sufficiently high pressure that the body of fluid may be driven through the reservoir where it collects and drives the reservoir oil to the production well.
The process of miscible flooding is extremely effective in stripping and displacing the reservoir oil from the reservoir through which the solvent flows. This effectiveness is derived from the fact that a two-phase system within the reservoir and between the solvent and the reservoir oil is eliminated at the conditions of temperature and pressure of the reservoir, thereby eliminating the retentive forces of capillarity and interfacial tension which are significant factors in reducing the recovery efficiency of oil in conventional flooding operations where the displacing agent and the reservoir oil exist as two phases in the reservoir.
More recently, carbon dioxide has been used successfully as a miscible oil recovery agent. Carbon dioxide is a particularly desirable material because it is highly soluble in oil, and dissolution of carbon dioxide in oil causes a reduction in the viscosity of the oil and increases the volume of oil, all of which improve the recovery efficiency of the process. Carbon dioxide is sometimes employed under non-miscible conditions, and in certain reservoirs it is possible to achieve a condition of miscibility at reservoir temperature and pressure between essentially pure carbon dioxide and the oil.
The use of carbon dioxide as a recovery agent by means of a conditional miscible flooding process, where the carbon dioxide miscibly displaces the reservoir oil is described in U.S. Pat. No. 3,811,502 to Burnett.
Other solubility or miscibility agents include nitrogen or light hydrocarbons and mixtures thereof, such as paraffins in the C.sub.2 to C.sub.6 range, and, in particular, liquid petroleum gas. Recent experience in oil recovery has made carbon dioxide an attractive fluid solubility or miscibility agent since it is more effective at lower pressures and it alleviates the need for natural gas and hydrocarbon solvents, both of which are in high demand.
Gaseous solubility or miscibility agents are also attractive if they are readily adapted to a continuous enhanced oil recovery system in which the injection of fluids into the injection well(s) and production of fluids from the production well(s) can be carried out without shutting in the production well(s) to allow pressure build-up or shutting in the injection well(s) to allow pressure draw-down at the production well(s). In most cases, high pressure is required to achieve miscibility, which, in the case of carbon dioxide, can be about 1000 to 4000 psig to effect direct miscibility with many oil systems. Due to the low viscosity of gas, however, gas injection systems experience early breakthrough and due to reservoir permeability stratification they may have poor injection profiles, resulting in reduced recovery efficiency. One method which has been tried to improve these systems is by alternating injection of water and gas.
In alternating injection of water and gas, the injected water preferentially invades zones previously swept by gas. Ideally, subsequent gas injections are diverted by water to zones not previously swept by the gas. However, since gas has a higher mobility than water, gas fingering occurs by which injected gas is not completely controlled to enter only the zones not previously gas-swept.
Furthermore, since the water is still mobile, and since in regions from which the oil has been removed the permeability to water may be increased, the water-to-oil production ratios may also be ultimately increased.
Surfactants can be included in the water to produce a gas foam to reduce gas mobility and lessen fingering but the problem of water mobility remains. Thickners can be added to the water to reduce water mobility but this increases the ratio of the mobility of the gas in relation to the water and can in fact make fingering worse
It is an object of the present invention to provide improved enhanced oil recovery using soluble or miscible fluids for recovery of oil deposits from mature oil reservoirs, which, among other things, overcomes some of the problems set forth above.